Abstract: Objective To evaluate the effect of different delivery modes on the pelvic diaphragm hiatus in postpartum women by transperineal real-time four-dimensional ultrasound.Methods From July 2015 to January 2017, 86 cases who gave birth in maternal and children's health care hospital of Hunan province were selected.Among them, 46 cases were natural childbirth and 40 cases were cesarean section.At the same time, 45 women without birth and abortion history were selected as the control group.All of the cases were evaluated the morphology of female pelvic diaphragmatic hiatus by transperineal four-dimensional ultrasound.Results Under resting state and Valsalvaaction, there was no significant difference in the left and right diameter, the anteroposterior diameter and the area of pelvic diaphragm hiatus between the natural childbirth group, the cesarean section group and the control group (P>0.05).Under the action of anal contraction, the left and right diameters, the anteroposterior diameter and the area of pelvic diaphragm hiatus in the natural childbirth group were (3.81±0.48) cm, (4.27±0.53) cm and (12.08±2.06) cm², respectively.They were significantly higher than those in the control group and the cesarean section group (P>0.05).Under the action of anal contraction, there was no significant difference in the left and right diameters, the anteroposterior diameter and the area of the pelvic diaphragm hiatus in the cesarean section group and control group (P>0.05).Conclusion Transperineal four-dimensional ultrasound can clearly observe the structure of the female pelvic floor.During the action of anal contraction, the pelvic diaphragm hiatus of vaginal delivery puerperais significantly increased.Under resting and Valsalva action, there was no significant relationship between the pelvicdiaphragm hiatus and the mode of delivery.